Control of chromium plating solutions



Patented June 2, 1936 PATENT OFFICE CONTROL OF CHROMIUM PLATINGSOLUTIONS Hiram S. Lukens, Philadelphia, Pa., assig'nor to UnitedChromium, Incorporated, New York, N. Y., a corporation of DelawareApplication February 3, 1982, Serial No. 590,688

. 27 Claims.

This invention relates to electrolytic solutions containing chromium anda catalyst and for use in electrolytic chromium plating, the inventionbeing more particularly directed to the control of the concentration ofthe catalyst in the plating solution.

In present practice satisfactory electrolytic deposits of chromium areproduced by passing a current from anode to cathode through anelectrolytic solution containing chromium in the presence of a suitablecatalyst, the object receiving the. deposit of chromium being used as acathode and the temperature of the solution, the anode current d ensity,the cathode current density and the respective concentrations of thechromium and of' the catalyst all needing to be kept within fairlywelldefined limits.

In continued use plating solutions have in the past tended toprogressively change with respect to the concentration of the chromiumand the concentration of the catalyst, tending to depart from, (fallbelow) the known limits essential for dependable satisfactory plating,and success in obtaining uniformly satisfactory chromium deposits hasthereforedepended not only upon careful control of temperatures andcurrent densities but also upon careful periodically repeatedadjustments in the concentrations of the plating solution with respectto chromium and with respect to the catalyst.

The control of the temperature of the solutions, the control of currentdensities and the control of chromium concentration can all beaccomplished by relatively simple and well defined processes. Thecontrol of the catalyst concentration has hitherto however presented amore complex problem.

It has hitherto been necessary periodically to examine the platingsolutions by more or less complex methods of analysis in order todetermine periodically the concentration of the catalyst and therebywhat additions of catalytic compound are necessary to keep theconcentration of the catalyst within allowable limits known to beessential for the production of uniformly satisfactory deposits.

I distinguish between the catalyst, which in solution is an ion havingcatalytic properties, and the substance in which the catalystis'supplied to the solution, and which for convenience I refer to as thecatalyst compound.

The different successfully used catalysts are acid radicals stable inthe bath and stable under the actions which occur during plating whencurrent is passing through the bath. Acid radicals conforming to theserequirements and that are known to be suitable for catalysts are thesulphate, fluoride, phosphate and borate radicals. These may be used, bysuitably introducing them in the form of one or more of the compounds ofthe catalyst with a metal or with a non-acid radical. I considerhydrogen as a. non-acid radical.

A purpose of this invention is to provide a simple and efiective meansfor keeping the concentration of the catalyst or catalysts in a platingsolution of the character indicated constant and within the limitsfavoring the production of satisfactory commercial chromium deposits.

A further purpose is to limit the maximum solubility of the catalystcompound, in a. chromium plating solution, and thus to limit the maximumconcentration of the catalyst (ion) in the solution to a value withinthe allowable range known to be essential for the production ofuniformly satisfactory deposits of chromium, and simultaneously toprevent depletion of the catalyst.

A further purpose is to supply the catalyst or catalysts from a compoundor compounds containing one or more other elements or positive radicals,said compound or compounds being distinguished by the fact that, ifadded in excess to electrolytic solutions containing chromium, enoughand only enough of the catalyst or catalysts ionize to provide a totalcatalyst concentration favorable to the production of satisfactoryelectrolytic deposits of chromium.

A further purpose is to insure the continuous presence of an adequateconcentration ofthe catalyst radical in a plating solution of thecharacter indicated by maintaining the solution in continuous contactwith an undissolved excess of a selected compound of the catalyst.

A further purpose Vis to maintain an'electrolytic solution of thecharacter indicated in a continuous saturation equilibrium with respectto a selected compound of the catalyst radical, using the solidcompound, in continuous contact with the plating solution, asa sourcefor an automatic supply of catalyst, andselecting a compound having theproper equilibrium solubility in the plating solution to give a catalystcancentration within the limits of concentration required for theplating.

A further purpose is simultaneously and automatically to avoid, in achromium solution, an excess of catalyst, received for example withmake-up chromium compound or with make-up water, and automatically toinsure an adequate catalyst concentration, keeping up the concentrationof the catalyst by maintaining the liquid in continuous contact with asolid compound comprising the catalyst acid radical and a selectedpositive radical or metal, and removing any excess catalyst byprecipitation by means of a compound containlng the said selectedpositive radical or metal. I

A further purpose is to use the limited solubility of a suitablecompound of a catalyst aut o-, matically to control the concentration ofthe catalyst within a chromium plating solution.

A further purpose is simultaneously to use temperature and thesolubility characteristics of a compound of a catalyst in anelectrolytic solution of a chromium compound to determine the content ofthe catalytic agent in the solution.

A further purpose is to maintains plating solution containing a chromiumcompound and a sulphate radical in continuous contact with solidstrontiumsulphate.

I A further purpose is to maintain a plating solu tion containing achromium compound and a sulphate radical in continuous contact withsolid strontium sulphate and upon the addition of make-up water or/andof make-up chromium compound to precipitate any excess sulphate radical,incident to any sulphate in the make-up water or/and in the make-upcompound, by means of a strontium compound other than strontiumsulphate, preferably strontium chrotion and in the claims.

The figure comprises solubility-temperature curves of a catalystcompound in a solution of a chromium compound adapted to form asatisfactory plating, liquor and to maintain in the liquor aconcentration of the catalyst within the requisite limits. The ordinatesare concentration in grams per liter and the abscissa are'degreescentigrade.

The curves are intended to show conventionally how to determine thesuitability or unsuitability of any compound of a catalyst for use inautomatic control of the catalyst concentration in a plating liquor, andthe specific curves also give the results of an actual experimentalinvestigation by the inventor to determine the adaptation of strontiumsulphate as a mediuni for applying the invention to a sulphate catalyst.The curves show the solubilities of strontium sulphate in solutions ofchromic acid of diiierent concentrations, and indicate the suitabilityof strontium sulphate'as a medium for controlling the concentration ofthe sulphate radical in chromium plating liquors represented by therange of chromic acid solutions indicated.

The catalyst in a chromium plating liquor has been found to compriseessentially an acid radical stable under conditions within the platingbath and in order to secure uniformly satisfactory deposits of chromiumit has been found that the concentration of the catalyst -in proportionto the concentration of chromic acid has to be maintained within fairlydefinite limits.

During the continued use of a plating liquor, the concentration of thecatalyst as well as the concentration of the metal containingconstituent in the liquor tend to change from difierent causes.

. Causes of such changes are:

1. Loss of solution adhering tothe objects plated on the removal of suchobjects from the plating bath and replacement of the lost solution bywater, or by water and the compound 0011- taining the metal constituentwhich is tobe plated or electrodeposited.

, 2. Consumption of the compound containing the metal constituent whichis electrodeposited or plated on the article during the process. 5

3. Replenishment of the bath with the compound containing the metalconstituent which is electrodeposited to replace the quantity consumedin electrodeposition.

4. Introduction of catalysts by compounds contained as impurities in thewater and in the compounds containing the metal constituent which is tobe electrodeposited, used for replenishment. v

These changes are not only absolute as to thecatalyst constituent, butrelative as between the catalyst constituent and the compound containingthe metal constituent which is to be electrodeposited.

In practice all of these factors are frequently material and variant andhave hitherto frequently resulted in an impossibility to definitelypredict the needed changes in the concentration of the catalyst 'exceptby troublesome and repeated analysis.

When the catalyst present as an impurity in the make-up materials hasbeen unduly high, these materialshave had to be purified from thecatalyst or from a portion thereof preparatory to their introductioninto the plating solution.

The object of the present invention is to correct automaticallysometimes for an excess of catalyst, added for example with the make -upwater or and with the make-up chromium compound, and sometimes for adeficiency of catalyst from the repeated carrying away of smallquantities of the catalyst in the plating liquor which adheres toobjects removed from the bath.

Let L represent the plating liquor without the catalyst and having achromium concentration 40 within the requisite limits for satisfactoryplating in the presence of the necessary concentration of catalyst.

Let C represent the catalyst, an acid radical. Let B represent any oneof the metals or non-acid radicals, B1, B2, B3 3;, eachof which isadapted to combine with C to form a compound BC definitely soluble in Lto a limited extent. If

a solid compound BC be added in excess to the liquor L, the liquordfssolves BC to saturation and in saturation equilibrium contains aconcentration of C determined by the maximum solubility of BC in L atthe temperature of the liquor.

Among the non-acid radicals B1, B2, B: Bl, let LB, represent a selectedmetal or non-acid radical B suited to the carrying out of the presentinvention with the liquor L.

- In the past the liquor L has always been undersaturated with anycompound BC added to provide a catalystC.

One of the more important features of my invention comprises a selectionand use from among the many available metals or non-acid radicals B of aradical B, such that the concentration of the catalyst in the liquor Lwhen determined by a complete saturation of the liquor with the compoundBiC will be within the known requisite limits of concentration forsatisfactory plating.

In accord with this feature of the invention, I maintain the platingliquor L in continuous contact with undissolved BQC and thereby incontinuous saturation equilibrium with 3,0, the saturation concentrationof B80 in the liquor L providing a concentration of C in .the liquorwithin the said limits of concentration for satisalways substantiallyfree from catalyst no catalyst' being added from any other source except33C, and the equilibrium or saturation concentration of B=C in L hasbeen reached, the selected metal or non acid radical B, being one togive a solubility or saturation concentration of C in the solution Lwithin the requisite limits of catalyst concentration, the correction\ofany change in concentration, by reason for exampleof carrying away ofthe plating liquor on plated objects,

is effected automatically by further solution from the excess solid BaC.

In the event that the make-up materials are not free from catalyst, thecatalyst may enter the solution L with the make-up chromium com-. poundor/and with make-up water. It is also possible that catalyst may beadded by mistake.

or in a misguided attemptto control the catalyst concentration byintroducing the-catalyst from a too-soluble compound.

The effect of such additional quantities of catalyst may obviously be toraise the quantity of catalyst beyond the requisite limits ofconcentration unless an additional means isprovided for preventing suchan increase in C.

' I prevent any undue increase in C by precipitating any such excess outof the liquor in "the form of BSC, and obtain this solid precipitationby adding to the liquor a compound BSR, which on solution yields thecommon ion B3. R is optionally any one of a number of -diflerent acidradicals which will not be harmful in the liquor. The compound BsR willreact in a strong acid solution of chromic acid to produce aprecipitation of BsCIO4-fi compound that can be only sparingly solublein the plating liquor.

The radical R will usually be preferably either the chromate radicalCrOr or the carbonate radical CO3. In most cases the radical R will bethe chromate radical.

As an example of the reaction that takes place when an excess of Centers the liquor with the make-up material, assume this radical hasbeen added in the form of a compound B'C.

The compound BC will react with the BsCI'O4 due to the added BSR, forexample in accord with the reaction:

tion of BC and therefore of -C in the liquor which corresponds to asaturated solution of 330. Therefore the concentration of BsC isunchanged except as the saturation concentration might be altered(somewhat reduced) by the presence of additional Bi cations incident toany of the excess BsCrO4 having gone into solution.

If B3 represents strontium, the B3CrO4 will be so nearly insoluble inthe plating liquor as to have little effect on the saturationconcentration of B30 in the plating liquor, leaving the C within therequisite limits of concentration for satisfactory deposition ofchromium, irrespective of whether or not the liquor is saturated withBsCrOi. as well as with BSC. It is therefore important to use a metal ornon-acid radical B; whose chromate is sparingly soluble.

4 very sparingly soluble,

This correction for the presence of catalyst in the make-up materialsmay be made with each addition of make-up materials, or more desirably,

L, the diiferent liquors L should preferably in-- clude not only thosehaving the requisite range of chromium contents, but should also bepreferably saturated with B1Cr04 before determining the saturationsolubility therein of BIG. This is true if the raw materials of theliquors in normal practice contain catalyst, or catalyst is dragged'intothe liquor as a result of the operation prior to plating, thereby makingit desirable to keep the liquor in continuous contact with the solidphaseof the chromate of the .selected metal or non-acid radical B, andis also normally unnecessary if the solubility of-B8CrO4 iii L is knownto be very small as compared to that of solid B86111 L. r

Any metal or non-acid radical Bi. may be used in carrying out myinvention if its compound B10, when dissolved to saturation in thechromium liquor L, alsosaturated with the compound B10104 gives aconcentration of B1C'such as to make C within the requisite limits ofconcentration for satisfactory deposition of chromium. Having consideredthe general principles, the application of my invention to a particularexample will be discussed briefly before considering vention.

To reach and preserve the lower limit of sulphate ion concentration, Iadd to the liquor an excess of strontium sulphate (Be is strontium),which dissolves and ionizes to just the proper extent to give a sulphateion (catalyst) concentration with respect to chromic acid within therequired range. As liquor is withdrawn or evaporates, and water isadded, more strontium sulphate dissolves, thereby building up thesulphate ion concentration automatically to the correct amount, as longas an excess of strontium sulphate remainsin contact with the liquor.

If sulphate ions be brought into solution, as for example as an impurityin the make-up materials, some strontium sulphate will be driven out ofsolution by common ion effect, and the resulting total sulphate ionconcentration will be less than the sum of the original sulphate ionconcentration, plus the sulphate ion concentra-.

tion due .to the impurity.

There is the possibility, however, that the sulphate ion concentrationwill be increased beyond the proper upper limit of sulphate ionconcentration. To prevent this, I add an excess of strontium chromate(BSR is here strontium chromate) to the liquor. The strontium chromateis and the chromate ion does volume of the solution.

no harm in any case, as the liquor is intended for plating chromium.

The strontium ions due to the strontium chromate lower the solubility ofstrontium sulphate by common ion eiIect, since they increase thestrontium ion concentration in the solubility product:

[Si-H504] 51-80 In the same way, sulphate ions brought in as ima platingsolution so concentrated with chromium as to be near the upper limits ofproper chromium concentration, and the curve B shows the solubiliy of Bin the solution after its chromium has been reduced by use to near itslower limit for uniformly satisfactory use.

The ordinates show the solubility, for example in grams of C per literof the plating bath while the abscissae represent temperature.Solubility is normally shown in grams per unit It is probable howeverthat in chromium plating'solutions a more direct criterion of theconcentration of the catalyst in the plating bath is represented by theratio between the weights of the catalyst and CrO: per

'unit of solution volume.

Though an aqueous solution of chromic acid for L and thesulphate radicalfor the catalyst have been described, because the ingredients arecommercially relatively cheap and free from impurities, a differentliquor for L or/and a different stable acid radical for the catalyst maybe used, in which even the metal or non-acid radical B, found to fulfillthe specified conditions might or might not be the same B; (strontium)suitable when L comprises an aqueous solution of chromic acid and Ccomprises S04. Likewise BiR, which is desirably strontium chromate whenthe liquor consists of chromic acid with a sulphate catalyst, may besome other substance when a different plating compound or/and difierentcatalyst is used. And for the sameplating compound and catalyst, anymaterials may be used for B, and BSR which meet the conditions.

Curve A specifically represents tests made by the inventor to determinethe solubility of strontium sulphate in an aqueous solution of chromicacid having a concentration of 400 grams per liter of CrO3.

Curve B represents the solubility of strontium sulphate in a chromicacid solution having a concentration of 200 grams per liter of CrOs, thesolubilities being shown in terms of grams of S04 (catalyst) per liter.

In using .a sulphate catalyst in plating solu- In practice it is fairlyeasy to maintain the temperature of the plating bath between theprescribed limits, for example between 40 and 50 C., or above, whetherthe solution is concentrated with respect to chromic acid or relativelydilute.

Referring to the curves, it will be seen that if the temperature weremaintained at 45 0., the ratio of S04 to CrOa in a solution saturatedwith strontium sulphate will be about .0056 for the conditionsrepresented by curve A and about .0109 when the. chromic acid contenthas reduced to the conditions of curve- B. These figures aresufficiently close to the optimum ratio for satisfactory operation. 4

In practice any small lessening of the range of permissible variation inthe concentration of the ClOa is of little importance as compared to theimportance of being able to avoid the requirement for chemical tests, todetermine the catalyst concentration incident to any such smalllessening of the full permissible range of variation in theconcentration of CrOz in the liquor.

In present practice it is possible'to obtain commercial chromic acidsubstantially free from catalyst,'and in view of this I may use thesolutions represented by the curves without first saturating the liquorwith strontium chromate or a similar compound. In fact, where puremake-up materials are available I will preferably employ only a compoundcontaining the catalyst to control the catalyst concentration.

In many cases however it will be more desirable to use the somewhatcheaper raw materials that contain a somewhat indefinite amount ofsulphate catalyst and, in this event, I prefer to keep the liquorcontinuously saturated with strontium chromate as well as saturated withstrontium sulphate.

Any such sulphate radical in excess of that needed to maintain theliquor L saturated with strontium sulphate reacts with the strontiumchromate and is precipitated as strontium sulremains parmanentlyundissolved, heat to 45 to 50 C. and start plating.

The concentration of the chromic acid solution may be determined withsufllcient accuracy by an hydrometer reading. After the chromic acidliquor has been used suiliciently to reduce the concentration of thechromic acid it is replenished by adding fresh acid, water also beingadded as needed, usually from any ordinary tap supply, the strength ofthe liquor with respect to chromic acid being determined suitably byhydrometer reading as before.

The strength of the catalyst is taken care of automatically, thestrontium sulphate dissolving until the new liquor is saturated withstrontium sulphate.

When the make-up materials contain more than traces of sulphate, or whensulphate is introduced by ml stakeor in a soluble compound, I add tothesolution some compound which will cause preoipltationofstrontiumsulphate in the liquor." This addition maybe an' excess ofstrontium chromate or may conveniently be strontiumfcarbonate It will,of course, be evident that the strontium sulphate need not be added assulphate.

If the raw materials contain sulphate radical in exc ss of that neededfor a production of a satura ed solution of strontium sulphate in theliquor L, I may simply make up the chromic acid 5 to the requisitestrength, as for example 250 to 300 grams per liter of C103. and addstrontium carbonate to an amount in some excess of that required for thepercipitation of the excess of sulphate radical, the result being atemporary evolution of carbon dioxide, precipitations of strontiumsulphate and strontium chromate in the bath and saturated solutions inthe bath of strontium sulphate and strontium chromate,-the concentrationof the strontium chromate however being verylow as compared to that ofthe strontium sulphate.

It willbe understood that the function of the solid strontium sulphatemaintained in continuous contact with the liquor is to insure in thebath a 9 concentration of sulphate as great as the minimum amountsuitable for uniformly satisfactory plating, that is to keep the bath upto standard with respect to the sulphate radical, and that the functionof the solid strontium chromate is to 25 maintain the sulphate catalystconcentration belowthe upper range permissible for uniformlysatisfactory plating with chromium, that is to keep the bath down tostandard with respect to the sulphate radical. 30 While one only of themetals or non-acid radicals adapted to use in an automatic maintenanceof the correct concentration of a catalyst has been discussed in full,the principles have been fully explained and can easily be applied tothe selection of other suitable metals or non-acid radicals for use withchromic acid as a platingmedium and with the sulphate ion as a catalystand with other plating media or/and other catalystswhatever the cost;and the claims are intended 40 to cover such other compound or compoundshaving the catalyst compound sparingly soluble the plating bath.

I may use variation or regulation in temperature of the plating bathwithin the permissible 45 limits of such variation to vary the weightratio between the catalyst and CrOs.

Thus if the plating liquor and catalyst are those adapted to bespecifically represented by the curves of the figure, if the temperaturefor the conditions of curve A is maintained at 45 C. and continuouslymaintained at 45 C. until the conditions of curve B have been reached,the variation in the weight ratio of C/CrOa will, as already stated, befrom about .0056 to .0109.

55 The .0056 ratio may readily be raised by increasing the temperatureabove 45" 0. when the conditions are those represented by curve A andthe ratio .0109 may readily be decreased by lowering the temperature asthe chromic acid becomes gradually depleted. For example if thetemperature were initially 50 C. under the conditions of curve A and 40C. under the conditions of curve B, the weight ratios, as indicated bythese curves, will be respectively .006 and .010. 65 It will beunderstood that such simultaneous variation oftemperature and weightratio may be made as the conditions of service require or renderdesirable.

It will be evident that, in the broad aspect of 70 my invention, Icontrol the catalyst content automatically. To do this, I maintain aplating solution, in which the liquid phase of the catalyst compound ispresent, continuously in contact with the solid phase of the catalystcompound. 7 It will be further evident that as a desirable,

' but not a necessary adjunct to my invention. I

control the solubility of the catalyst compound by common ion effect. Ipreferably do this by maintaining the solid phase of a sparingly solublecompound containing the common ion in con- 5 tinual contact with theliquor.

While I have discussed rather fully the inhibiting of the catalystsolubility by common ion effect, it will be evident that my inventioncontemplates the use of any desired and suitable means for 10 limitingthe upper range of catalyst concentration, no matter what the inhibitingagent and without regard to the mechanism by which it acts upon thecatalyst to limit the catalyst concentration. 15

The specific examples given have been largely limited to the employmentof the sulphate ion as a catalyst, but, as pointed out, other ions suchas the fluoride, borate and phosphate ions, are capable of use. It isalso possible to employ several catalyst ions concurrently, and inthiscase if the upper range of catalyst concentrations is to be limited, Iwill either employ a single compound which restricts the concentrationsof the various catalysts present or use separate compounds to inhibitthe solubilities of the different catalysts. In any case, the compoundwhich inhibits the catalyst solubility must be specially selected withreference to the catalyst itself and to the chromium compound from whichchromium is to be plated.

In view of my invention and disclosure variations and modifications tomeet individual whim or particular need will doubtless become evident toothers skilled in the art, to obtain all or part of the benefits of myinvention without copying I my specific disclosure, and I, therefore,claim all such in so far as they may fall within the reasonable spiritand scope of my invention.

plating liquor containing chromic acid within a desired range ofconcentration, which consists in keeping the liquor in continuouscontact with the solid phase of a selected compound of the catalyst acidradical and a metal or non-acid radical, the solubility of the compoundin the plating liquor being such as to give the liquor, when saturatedwith the compound, a catalyst content within the desired range. i k

2. The method of maintaining the concentration of an acid radicalcatalyst in a chromium plating liquor containingchromic acid to adesired range of concentration, whichconsists in keeping the liquor attemperatures within a selected range thereof and in continuous contactwith the solid phase of a selected compound of the catalyst acid radicaland a metal or non-acid radical, the solubility of the compound in theplating liquor exclusive of the catalyst being such as to give theliquor, when at the said tempera- I tures and saturated with thecompound, a catalyst content within the desired range of concentration.

3. The method of maintaining the concentration of an acid radicalcatalyst in a chromium plating liquor containing chromic acid within adesired range of concentration, which consists in \keeping the liquor attemperatures within a selected range thereof and in continuous contactwith the solid phase of a selected compound of the catalyst, saidcompound comprising the catathe solubility of the compound in the,plating liquor being such as to give theliquor, when at and saturatedwith the.

the said temperatures compound, a catalyst content within the desiredrange of concentration and in adjusting the temperature to accommodatechange of concentration of the liquor with respect to chromium.

4. The method of maintaining a permissible range of concentration ofcatalyst in a chromium plating liquor containing chromic acid, whichconsists in maintaining the liquor in contact with an excess of acompound which yields the catalyst. upon dissolving and in limiting thesolution of the cataLvst by common ion effect.

5. The method of maintaining the concentration of an acid radicalcatalyst in a chromium plating liquor containing chromic acid within adesired range of concentration, which consists in keeping the liquor incontinuous contact with the solid phase of a selected compound, the saidcompound comprising the chromate of a selected metal or non-acidradical, the said metal or nonacid radical being adapted to combine withthe catalyst acid radical to form a second compound having a solubilityin the plating liquor such that, when the liquor is saturated with thesecond compound, the catalyst portion thereof will have a concentrationwithin the said range of catalyst concentration.

6. The method for maintaining the catalyst.

range of catalyst concentration. for satisfactory plating and in keepingthe solution down to a standard with respect to its catalystcontent bymaintaining the liquor in continuous contact with the solid phase of achromate of the said metal or non-acid radical. v

'7. The method of maintaining the concentration of an acid radicalcatalyst in a chromium plating liquor containing chromic acid to adesired range of concentration, which consists in keeping the liquor attemperatures within a selected range thereof and in continuous contactwith the solid phase of a selected compound, the said compoundcomprising the chromate of a selected non-acid radical, the saidnon-acid radical being adapted to combine with the catalyst acid radicalto form a second compound having a solubility in the plating liquor suchthat, when the liquor is at the said temperature and saturated with thesecond compound, the catalyst portion thereof will have a concentrationwithin the said range of catalyst concentration.

8. The method of maintaining the concentra-- tion of an acid radicalcatalyst in .a chromium plating liquor containing chromic acid within adesired range of concentration, which consists in keeping the liquor attemperatures within a selected range thereof and in continuous contactwith the solid phase of a selected compound, the said compoundcomprising the chromate of a selected nonacid radical, the said non-acidradical being adapted to combined with the catalyst acid radical to forma second compound having a solubility in the plating liquor such that,when the liquor is at the said temperatures and saturated with thesecond compound, the catalyst portion thereof will have aconcentrationwithin the said range of catalyst concentration, and v in adjusting thetemperature to accommodate change of concentration of the liquor withrespect to chromium;

9. The method of controlling the concentration of sulphate ionsin achromic acid chromium plating liquor, which consists in dissolvingstron- 'tium sulphate in the liquor and in further dissolving in theliquor a compound anf strontium free from sulphate. 7

10. The method of controlling the concentration of sulphate ions in achromic acid chromium .15 plating liquor, which consists'in maintainingan excess of strontium sulphate in contact with the liquor and infurther maintaining an excess of a strontium compound other thanstrontium sulphate in contact with the liquor. 1

11. The method of controlling the concentration of sulphate ions in achromic acid chromium plating liquor, which consists in maintaining anexcess of strontium sulphate and an excess of strontium chromate in theliquor.

12. The method of controlling the catalyst content of a chromium platingliquor containing 4 chromic acid which consists in maintaining cationsin controlled excess of the catalyst anions, selecting and maintainingin the bath a sparingly soluble substance containing a cation honacidradical adapted to combine and precipitate any excess of the catalystanions.

13. The method for keeping the catalyst content of a chromiumplating-liquor containing 35 chromic acid to anamount within a standardrange notwithstanding the addition of catalyst to an amount that wouldif uncorrected raise the catalyst concentration above the range, whichconsistsin dissolving a catalyst compound in the liquor to substantialsaturation, the said compound comprising the catalyst acid radical and aselected non-acid radical and in supplying the liquor with the non-acidradical as cations in controlled excess of the catalyst anions.

14. A chromium plating liquor comprising a solution of chromic acid. asparingly soluble catalyst compound present in excess, yielding acatalyst ion within the desired range for plating and another ion, and asparingly soluble compound present in excess yielding the other ion freefrom the catalyst ion.

15. A chromium plating liquorcomprising a chromic acid solution,strontium sulphate present in excess and strontium chromate present inII excess. v

16. As a bath for electrolytic deposition of chromium, a solution ofchromic acid with an addition agent adapted to promote the deposition ofchromium, such addition agent being a compound which is soluble atplating temperatures in the chromic acid solution only to the extentwhich affords a working concentration of the addition agent in solution.there being in the bath an undissolved excess of such addition agent.

17. As a bath for electrolytic deposition of chromium, a solution ofchromic acid with an addition agent adapted to promote the deposition ofchromium, such addition agent being a compound which is soluble atplating temperatures in the chromic acid solution only to the extent 4which affords approximately the optimum concentration of the additionagent in solution, there 7 being in the bath an undissolved excess ofsuch addition agent.

18. As a bath for electrolytic deposition of chromium, a solution ofchromic acid with a strontium compound as an addition agent to promotethe deposition of chromium, the strontium compound being soluble atplating temperatures in the chromic acid only to the extent whichaffords a working concentration of the strontium compound in solution,there being in the bath an undissolved excess of the strontium compound.

19. As a bath for electrolytic deposition of chromium, a solutioncontaining approximately 250 to 300 grams per liter of chromic acid witha strontium compound as an addition agent to promote the deposition ofchromium, the strontium compound being soluble at plating temperaturesin the chromic acid only to the extent which affords a Workingconcentration of the strontium salt in solution, there being in the bathan undissolved excess of the strontium compound.

20. As a bath for electrolytic deposition of chromium, a solutioncontaining approximately 250 to 300 grams per liter of chromic acid andstrontium sulphate in excess of the amount thereof soluble in thechromic acid.

21. As a bath for electrolytic deposition of chromium, a solution ofchromic acid containing strontium sulphate in excess of the amountthereof soluble in chromic acid.

22. The method of depositing chromium electrolytically which compriseselectrolyzing a bath containing chromic acid and an excess of anaddition agent which promotes the deposition of chromium and which issoluble at plating temperatures to the extent only of providing in thechromic acid solution. a working concentration of the addition agent insolution and permitting solution of the excess of the addition agent tomaintain the working concentration.

23. The method of depositing chromium electrolytically which compriseselectrolyzing a bath containing chromic acid and an excess of, anaddition agent which promotes the deposition of chromium and which issoluble at plating temperatures to the extent only of providing in thechromic acid solution approximately the optimum concentration of theaddition agent in solution and permitting solution of the excess of theaddition agent to maintain the optimum concentration.

24. The method of depositing chromium electrolytically which compriseselectrolyzing a bath containing chromic acid and an excess of astrontium compound as an addition agent, which promotes the depositionof chromium and which is soluble at plating temperatures to the extentonly of providing a working concentration of the addition agent insolution and permitting 'sotrolytically which comprises electrolyzing abath containing 250 to 300 grams per liter of chromic acid and an excessof strontium sulphate as an addition agent and permitting solution ofthe excess of the addition agent to .maintain the concentration of theaddition agent.

27. The method of depositing chromium electrolytically which compriseselectrolyzing a bath containing chromic acid and an excess of astrontium compound as an addition agent, which promotes the depositionof chromium and which is soluble at plating temperatures to the extentonly of providing approximately the optimum concentration of theaddition agent in solu-' tion and permitting solution of the excess ofthe addition agent to maintain the optimum concentration.

' HIRAM S. LUENS.

